I guess you refer to the stickers category in the part library? There are a lot of stickers to choose from, but they are already attached to the bricks and you'll find them under each brick category if you click on the + sign. Only official LEGO stickers have been added so far but in the future you'll be able to add your own custom stickers/decorations.

Oooh I see, but I have seen LEGO sets with already textured bricks that are not in the editor so it would be nice to have.

You may have seen renders made by users in Blender where you can change the decorations to you own creations? But right now, that is not possible in Mecabricks workshop and player. Scrubs is working on it though...